Since orthogonal frequency division multiplexing (OFDM) is a multi-carrier transmission technique, the available spectrum is divided into many sub-carriers, each being modulated by data at a relatively low data rate. OFDM can support multiple access by allocating different sub-carriers to different users. The sub-carriers for OFDM are orthogonal and closely spaced to provide an efficient spectrum. Each narrow band sub-carrier is modulated using various modulation formats, such as quadrature phase-shift keying (QPSK) and quadrature amplitude modulation (QAM). OFDM modulation is provided using an Inverse Fast Fourier Transform (IFFT). Initially, data for transmission is mapped into quadrature-based symbols that are encoded onto the individual sub-carriers. An IFFT is performed on the set of modulated sub-carriers to produce an OFDM symbol in the time domain. Typically, a cyclic prefix is created and appended to the beginning of the OFDM symbol before it is amplified and transmitted. During reception, the OFDM symbols are processed using a fast Fourier transform (FFT) to recover the modulated sub-carriers, from which the transmitted symbols can be recovered and decoded to arrive at the transmitted data.
As noted, to facilitate multiple user access, data for transmission is allocated to groups of adjacent sub-carriers, wherein these groups remain consistent from one OFDM symbol to the next. With reference to FIG. 1, each circle represents a sub-carrier for a sequence of OFDM symbols. Each row represents the sub-carriers associated with an OFDM symbol, and each OFDM symbol is transmitted in sequence over time. In this example, users 1 and 2 require a voice service, wherein users 3 and 4 require data and video services, respectively. The voice services require lower data rate than the data services, while the video service requires the most resources. As such, the groups of sub-carriers dedicated to voice, such as that for users 1 and 2, are less than that for users 3 and 4. User 4 is using as much of the spectrum as the first three users combined. Notably, along the time-frequency plane for the OFDM spectrum, the mapping of user data to various sub-carriers is repetitive and consistent. Due to the significant variations in the communication channel, especially for the frequency selective fading channel, and interference over the time-frequency plane, such multiple access mapping results in a different carrier-to-interference ratio for each user. The different carrier-to-interference ratios will lead to unequal degradation of performance for each user.
In an effort to minimize the impact of the variations in the channel, frequency-hopping schemes have been employed to systematically remap the groups of sub-carriers associated with each user to different points in the time-frequency plane, as illustrated in FIG. 2. Thus, users are assigned one or more transmission blocks consisting of a set number of sub-carriers within a set number of adjacent OFDM symbols. Thus, a user does not necessarily transmit on the same sub-carrier group for every OFDM symbol, but will jump to a different sub-carrier after a period of time based on the defined hopping pattern. The sub-carrier hopping scheme illustrated in FIG. 2 improves the performance over the fixed time-frequency allocation illustrated in FIG. 1; however, the performance could be further improved if the diversity across the whole band were fully exploited.
Most solutions proposed to reduce the interference in frequency-hopped systems are based on the assumption that the different interfering transmitters are synchronized through a global positioning system (GPS) or the like. These solutions are not applicable to communication systems that are not synchronized, such as Universal Mobile Telecommunications System (UMTS).
Other frequency hopping schemes are based on non-synchronized transmitters, but they usually use different pseudo-random hopping sequences, with no way to discriminate the interference level for separate receivers. Hence, a receiver experiencing a low carrier-to-interference ratio will get the same probability of sub-carrier collisions as a receiver with a high carrier-to-interference ratio. This is not optimal, since the high-carrier-to-interference ratio receiver does not necessarily need to avoid collisions as much as a low carrier-to-interference ratio receiver. Thus, there is a need for an efficient sub-carrier mapping technique to minimize the impact of channel variations and interference over the time-frequency plane.